I always recommend to use the same thread, specially the same color on the top and bottom. You can adjust your machine to have the perfect tension for that particular thread or fabric or sandwich in general, but even the best machine hiccups every once in a while and your bobbin thread will show a bit. If you are using the same color in the bobbin, you will not even notice. I always prepare a practice little sandwich to make sure there are no bird nests in the bottom. Sometimes we try to match the top to the top and the bottom to the color of the bottom fabric, but I prefer to make sure that my top looks good. After all, that is the part you see first and most of the time. I tend to make wallhangings a lot, and I don't really care about matching the color on the back.